Vehicles with the Auto Ride option the shocks will have an air line hooked up to it, and sometimes an electrical connector (they are also usually painted orange, and have a rubber bladder wrapped around the middle). This is with the rear shock with the electrical connector and air line hook up would look like on the drivers rear.

You would also have an air compressor located ont he left frame rail, just behind the left rear wheel:

Also there is usually a badge on the truck just behind the rear doors, you should have an "LT" or "SLT" vehicle model designation. If you have Auto Ride, it usually says "Auto Ride" on the badge as well. If you still are unsure, call the GM dealership and give them your VIN number and they'll be able to tell you if your vehicle is equipped with this option.